Some problems of the tectonic framework of the Guiana Shield with special regard for the Roraima Formation

Zusammenfassung Die Arbeit diskutiert Probleme der tektonischen Entwicklung des Guyana-Schildes, eines gro\en kratonischen Gebietes archaischen Alters. Seine Konsolidation fand viel früher statt als die Konsolidation des Brasilianischen Schildes. Guyana stabilisierte sich schon wÄhrend des Älteren Proterozoikums vor 1800 M. J. Vulkanite und Molasse der Roraima-Formation füllten die Depressionen der gefalteten archaischen Gebiete auf. Der Sockel des Guyana-Schildes wird durch die Guyano Eburnéenne Geosynclinal (Choubert, 1969), die Transamazonische Geosynklinale, gebildet, welche sich von Venezuela bis Französisch-Guyana mit gleichem Streichen über eine Entfernung von mehr als 1000 km verfolgen lÄ\t. Eine archaische Geosynklinale von diesem Ausma\ ist bisher aus anderen Schildgebieten der Erde unbekannt.Die archaischen Metasedimente, Metavulkanite und kristallinen Gesteine von Guyana haben allgemein eine E—W-Streichrichtung und unterscheiden sich damit von der vorherrschenden N—S-Streichrichtung der oberprÄkambrischen Gesteine des Brasilianischen Schildes.Jung-prÄkambrische Geosynklinalen und Orogenesen scheinen im Guyana-Schild zu fehlen. Die in Brasilien aktive jung-proterozoische Assynthische (Baikalische oder Brasilianische) Orogenese hat den Guyana-Schild nicht erfa\t. Dort war die letzte Orogenèse die Post-Barama-Mazaruni-Bartica-Orogenese vor 2000 M. J. Zwischen 2000 M. J. und 1800 M. J. entwickelte sich die PrÄ-Roraima-ErosionsflÄche. Dieser Phase folgte die Ablagerung der tafeligen Roraima-Formation vor 1750 M. J. mit dem anorogenen Post-Roraima-Vulkanismus (Dolerite und Gabbro-Intrusionen). Die gro\e Verbreitung der fast nicht verformten Roraima-Sedimente beweist, da\ der grö\te Teil des Guyana-Schildes wÄhrend des Älteren Proterozoikums stabilisiert war.Das Problem der Entstehung des Amazonas-Beckens und die Rolle der Transcurrent-Verwerfungen sowie das VerhÄltnis zwischen den kontinentalen Transcurrent-Verwerfungen und den Bruchzonen des mittelozeanischen Rückens werden dargestellt.

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In this paper some problems of tectonical evolution of the Guiana Shield are discussed that represents a large cratonic area of Archaic age. Its consolidation took much earlier place than the consolidation of the Brazilian Shield. The Guianas were already settled at early Proterozoic time, 1.800 m. y., when molasse and volcanic deposits had been accomplished at depressions of areas folded during the Archaic time 2.000 m. y. The basement of the Guiana Shield is formed by rocks of the vast Guiana-Eburneana Geosyncline (Choubert, 1969), named also Trans-amazonian Geosyncline which has an extension of more than 1.000 km, with the same direction. Such a huge Archaic geosyncline is unknown at an other cratonic area of the world.The predominant trend and structural lineament of the Archaic rocks is quite different than in the Brazilian Shield. The characteristic trend of the early Precambrian rocks of the Guiana Shield is approximately E-W, contrary to the main N-S structural alignement of the Brazilian Shield.Late Precambrian geosynclines and orogeny seem to be absent in the Guiana Shield. The young Proterozoic Assynthic (Baicalian) Orogeny has not more affected the Guiana Shield, where the last orogeny was the Post-Barama-Mazaruni-Bartica Orogeny, which occurred at the Archaic, 2.000 m. y.Between 2.000 m. y. and 1.800 m. y. took place the development of Pre-Roraima erosion surface. It was followed by the deposition of the tabular Roraima Formation, manifesting the stabilisation of the central part of the Shield before 1.750 m. y., when the emplacement of the gabbroic and doleritic volcanites succeeded. The large distribution of the not metamorphosed flat lying Roraima sediments proves that the Guiana Shield was already consolidated during the old Proterozoic time and was never more affected by younger orogenic movements.It is also discussed the problem of origin of the Amazonas Basin, and the important role of the Transcurrent Faults in the tectonic framework of the Guiana Shield and the probable genetic relationship between the transcurrent faults and the fracture zones of the Mid-Atlantic Ridge.

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Resumo O Escudo das Guianas representa uma ampla área cratÔnica da Era Arcaica. Sua consolidaÇÃo já ocorrera muito antes de consolidaÇÃo do Escudo Brasileiro. As Guianas tornaram-se estáveis durante a Era Proterozóica inferior, 1.700 m. a. quando deposiÇÕes vulcânicas e molasse de Roraima foram concluidas em depressÕes de áreas dobradas durante a Era Arcaica, O embasamento do Escudo das Guianas é formado pelo imenso Geossinclínio Guiano-Eburneano (Choubert, 1969) ou Transamazoniano, que representa um geossinclinal de mais de 1.000 km de extensÃo, de mesma direÇÃo. Geossinclinal Arqueano de tamanha amplitude nÃo é conhecido em outra regiÃo cratÔnica de mundo.As rochas metassedimentares, metavulcânicas e cristalinas arcaicas tÊm um alinhamento aproximadamente E-W, diferindo do principal alinhamento estrutural N—S do Escudo Brasileiro.Geossinclínos e orogÊnia do Pré-Cambriana superior parecem estar ausentas no Escudo das Guianas. A orogénia do Proterozóico superior (Assíntica ou Baicaliana) nÃo mais afetou o Escudo das Guianas, onde a Última orogÊnia foi a orogÊnia Barama-Mazaruni-Bartica, que ocorreu há 2000 m. a. Entre 2000 m. a. e 1800 m. a. deu-se o desenvolvimento da superfície de erosÃo Pré-Roraima. Esta fase foi seguida pela deposiÇÃo da FormaÇÃo Roraima tabular, o que revela a estabilizaÇÃo da parte central do Escudo, há 1.750 m. a. quando ocorreu o emplacement dos vulcanitos gabróicos e doleríticos. A grande distribuiÇÃo dos sedimentos Roraima quase horizontals nÃo metamorfizados, prova que o Escudo de Guianas já estava consolidado durante o Proterozóico inferior e nÃo foi mais afetado por movimentos orogÊnicos mais jovens.Foi discutido também o papel importante das falhas transcorrentes na constituiÇÃo tectÔnica do Escudo das Guianas; o problema da origem da Bacia AmazÔnica, e a provável relaÇÃo genética entre as falhas transcorrentes as zonas de fratura da Cadeia Meio-Atlântica.

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Some aspects of sandwiched soil cover design for toxic waste repositories

The environmental concern favours “dry” waste management systems compared to the more common practice of shallow land burial. With the former, sandwiched soil covers are usually employed. A double barrier concept is presented and discussed for the sandwiched construction. The critical matrix potentials of the two barrier materials appear to have significant influence on the performance of the dual barrier system. Despite some practical problems, the concept appears promising.